1. Field of the Invention
The instant disclosure relates to a sensor in a mouse; in particular, to the sensor utilizing an external clock frequency to automatically adjust internal clock frequency.
2. Description of Related Art
An optical mouse sensor captures images at a certain frame rate and computes the displacement between images through image comparison. The frame rate is determined by a master clock that is typically generated internally in the sensor. The frequency of the internal clock source will vary over process, voltage and temperature variation. Since the logic circuit that determines the frame rate is driven by the same clock source, the frame rate will also vary in the same way. The internal clock source variation can be as large as +/−20%.
The performance of the mouse is sensitive to frame rate. Two examples of such performance parameters are maximum speed and maximum acceleration. The maximum speed and acceleration of a mouse increases as its frame rate increases. Therefore, to guarantee that a mouse is able to meet a given speed and/or acceleration specification, the variation of the internal clock source needs to be taken into account. For example:
Let the specification be 10 ips maximum speed and 1 g maximum acceleration.
To meet the specification, let the minimum frame rate be 1000 fps (frames per second).
Given a nominal clock frequency of 10 MHz and a +/−20% clock frequency variation, the min-typ-max clock frequency is 8-10-12 MHz.
In order to meet the speed and acceleration specification over the range of clock frequency variation, we need to set the minimum frame rate i.e. the frame rate at 8 MHz to be 1000 fps.
Therefore, the min-typ-max frame rate is 1000-1200-1440 fps, given the min-typ-max clock frequency of 8-10-12 MHz.
In the example above, we can guarantee that, despite variation in clock frequency, any given mouse will meet the 10 ips and 1 g specification because the frame rate is designed to >1000 fps over the clock frequency variation.
However, power consumption increases almost linearly with frame rate. Therefore, sensors with a frame rate above 1000 fps consume more power than what is needed to minimally meet the 10 ips and 1 g specification.